The present invention relates to a torque converter to which a hydraulically actuated lockup clutch is attached.
The torque converter includes an impeller shell which is mounted to a front cover linked to an engine output shaft and is provided with a pump impeller, and a turbine runner connected to a main shaft so as to be opposed to the pump impeller, and serves as a power transmission apparatus amplifying an engine output and transmitting thereof to the main shaft serving as an output shaft of the torque converter. Since the torque converter transmits the engine output by using fluid, i.e., oil as a medium, a power transmission loss caused by a slip of the fluid is unavoidable. Accordingly, in a range in which a rotational speed difference between the pump impeller and the turbine runner is small, the engine output shaft and the main shaft are set in a directly coupled state by actuating the lockup clutch so that the engine output is directly transmitted to the main shaft.
The lockup clutch is connected to the main shaft, and is actuated to an engagement state, in which the lockup clutch is engaged with a front cover, i.e., a converter cover to directly connect the engine output shaft to the main shaft via the front cover, and a disengagement state, in which the engagement is cancelled. The operation of the lockup clutch is controlled by utilizing the oil that is a power transmitting medium. In other words, the operation of the lockup clutch is controlled by utilizing a pressure difference between the oil supplied into a hydraulic chamber formed between the converter cover and the lockup clutch, and the oil supplied into a converter chamber accommodating the turbine runner.
Such a torque converter that a multiple disc clutch is used as the lockup clutch has been known. For example, in a technique disclosed in Patent Document 1 (Japanese Patent Laid-open No. 2001-116110), the lockup clutch serving as the multiple disc clutch and a lockup piston for engaging the lockup clutch are arranged in the lockup chamber communicating with the converter chamber. The lockup piston is constantly pressed in a direction of canceling the engagement of the lockup clutch due to the hydraulic pressure of the oil flowing in the lockup chamber. However, in the region in which the above-mentioned rotational speed is small, the lockup piston can be operated in the direction of engaging the lockup clutch by supplying the oil into the hydraulic chamber formed between the converter cover and the lockup piston.
The above lockup clutch is also mounted on a hybrid vehicle using, as a power source of a vehicle, an internal combustion engine such as a gasoline engine together with an electric motor. As an example, in a technique disclosed in Patent Document 2 (Japanese Patent Laid-open No. 2002-195376), the lockup clutch is arranged between a damper and an inner peripheral wall of the converter cover.
As mentioned above, the conventional lockup clutch actuated by the hydraulic pressure is formed so as to be open to an inside of the converter chamber in which the torque is amplified, and is always affected by the hydraulic pressure within the converter chamber. Accordingly, in the techniques disclosed in the Patent Documents 1 and 2, since the hydraulic pressure within the converter chamber is applied as a reaction force to a pushing force of the lockup piston in engaging the lockup clutch, it is necessary to make an engagement hydraulic pressure high or increase a pressure receiving area of the lockup piston to which the engagement hydraulic pressure is applied for ensuring a predetermined engaging force. If the engagement hydraulic pressure is made high, a load of the oil pump is increased, so that this is undesirable in view of improving a transmission efficiency. In the techniques disclosed in the above Patent Documents 1 and 2, so the clutches have such a structure that the small-diameter multiple disc clutch is arranged at a comparatively far position from the main shaft and the pressure receiving area of the lockup piston is enlarged.